From the prior art it is known to use dispersions based on copolymers in water-thinnable paints and coating materials. Where the aim is for a high quality level, with respect to, for example, the resistance of the films to solvents or chemicals, the dispersions are equipped with crosslinkable functional groups and prior to application are mixed with a crosslinker, for example with a polyfunctional isocyanate. Aqueous two-component polyurethane coatings, including dispersions based on copolymers containing groups that are crosslinkable with isocyanates, and including polyfunctional isocyanates, are increasingly being used for such profiles of requirements, on the basis of their high quality level.
EP-A 557 844 describes aqueous two-component polyurethane coatings based on hydrophilic polyisocyanates and hydroxy-functional emulsion copolymers which are practically free of carboxylate groups and are stabilized by means of external emulsifiers. Owing to the high molecular weights of the polymers and the enduring hydrophilicity when external emulsifiers are used, such coating systems, for certain applications, have disadvantages in water resistance, pigment wetting, and, in particular, in the optical qualities of the film.
EP-A 358 979, DE-A 42 262 70 and EP-A 1 024 184 describe aqueous two-component polyurethane reactive systems based on vinyl polymer secondary dispersions and polyisocyanate crosslinkers which already have a good level of properties. The vinyl polymers in the systems serve as emulsifiers for the polyisocyanates and so prevent the unwanted reaction between NCO and water. For certain applications, however, improvements in the solids content, in the resistance properties, particularly the water resistance, in film hardness, and in respect to the optical qualities of the film are desirable.
DE-A 44 35 950, DE-A 44 07 415, and DE-A 44 39 669 describe not only solventborne but also aqueous coating materials, in which the drying and also the chemical resistance and/or weather resistance of the coatings can be improved by additionally using cycloaliphatic monomers. For many applications, however, a further improvement in film hardness and—in the case of waterborne coating materials—in particular in the optical qualities of the film (gloss, haze) is desirable.
Disadvantages in the optical qualities of the film, such as low gloss, appreciable haze, levelling problems, for example, are observed in particular when increased use is made of what are referred to as “hardening monomers”, i.e. those monomers, which in the form of a homopolymer, have a glass transition temperature (Tg) of more than 0° C. Examples of “hardening” monomers are styrene, methyl methacrylate, n-, iso- or tert-butyl methacrylate, and cycloaliphatic (meth)acrylic esters. The cause of these deficiencies in the optical qualities of the film is the difficult or poor incorporability or dispersability of the polyisocyanate crosslinkers in the polyol dispersion, leading to a more heterogeneous distribution of the crosslinker in the binder.
It was an object of the present invention to provide aqueous binders and the coatings resulting therefrom, featuring improved film hardness and optical qualities, which do not have the disadvantages of the prior art.